A conventional solar thermal electric power generation system 101 shown in FIG. 25 has a configuration wherein: sunlight is collected by means of a concentrating type heat collecting unit (hereinafter will be referred to as “heat collecting unit” simply) 102; a heating medium absorbs collected sunlight as thermal energy; and the heating medium is supplied to a heat exchanging device 103 to generate steam by utilizing heat of the heating medium. Saturated steam generated by the heat exchanging device 103 is then superheated by means of a superheater 104. A steam turbine 105 is driven by such superheated steam to generate electricity. In the figure, reference characters 106 and 107 denote an electricity generator and a condenser, respectively.
Methods of concentrating incidental solar radiation are roughly classified into the central receiver type, the dish type and the parabolic trough type. The heat collecting unit 102 of the parabolic trough type uses trough-shaped reflectors 102a having a parabolic section in an X-Y plane and configured to reflect sunlight thereon to concentrate the sunlight on its focal point. Heat absorbing tubes 108 with high absorptivity of solar heat each extending through the focal points of reflectors 4a along the Z-axis allow a heating medium to flow therethrough to collect solar radiation. The heat absorbing tubes 108 and heating medium supply piping 109 connected thereto make circulating flow of the heating medium between a heat exchanging device 7 and the heat collecting unit. A special synthetic oil is generally used as the heating medium. The heating medium absorbs, for example, solar heat to reach a high-temperature condition of about 400° C., releases the heat and generates steam in the heat exchanging device 103 to assume a low-temperature condition of about 300° C., and returns to the heat collecting unit 102.
As can be seen from FIG. 26 plotting the solar energy density varying during one day, the conventional solar thermal electric power generation system can operate only during daytime from sunrise to sunset. For this reason, the system is stopped at night and must be restarted in the next morning. FIG. 26 plots the solar energy density varying during one day at a region in North Africa. Curves plotting mean energy densities in July and December are shown respectively in FIG. 26, and curves plotting mean energy densities in other months are considered to fall within the range between the two curves.
As shown, the intensity of solar thermal energy reaching the heat collecting unit 102 varies from zero to maximum during one day. Therefore, the electric power generation system 101 is usually designed to have such a capacity as to generate electricity at a mean solar energy intensity level. As is often the case, the system 101 is designed to store surplus energy in excess of a mean solar energy intensity level as thermal energy in a large-scale and expensive heat storage system 110 and release the heat thus stored to generate steam thereafter, thereby making it possible to continue electric power generation. Actually, however, limitations on the system investment cost and running cost limit the heat storage capacity to about 4 to 6 hours in terms of electric power generating duration and, therefore, electric power generation cannot be continued all day long.
In an attempt to solve this problem, an integrated solar combined cycle electric power generation system has been proposed which combines the above-described steam turbine electric power generation relying upon only solar heat with the gas turbine combined cycle electric power generation (see patent documents 1 and 2 listed below for example). Such a new concept of solar thermal electric power generation system is intended to generate electricity, even during night-time or cloudy days during which solar heat cannot be utilized, by a combination of the gas turbine electric power generation system with the steam turbine electric power generation system by utilizing steam generated in a waste heat recovery boiler. The system thus configured can be expected to continue electric power generation through day and night. Also, the integrated system can be expected to reduce the fuel consumption of the gas turbine, and hence, reduce the carbon-dioxide emission amount, by utilizing solar heat during daytime to a maximum extent.
However, another type of integrated solar combined cycle electric power generation system includes a heat collecting unit configured to generate saturated steam directly from water and supply it to the steam turbine without using a special heating medium and a heat exchanging device. This kind of electric power generation system according to patent document 1 is configured to mix the saturated steam with the steam generated from a high-pressure turbine for superheating the saturated steam before supplying the steam to the steam turbine. On the other hand, another kind of the electric power generation system according to patent document 2 is configured to mix the saturated steam with the steam generated from a high-pressure turbine and then superheat the saturated steam by means of a reheater of the waste heat recovery boiler before supplying the steam to the steam turbine.
Regardless of whether the conventional solar thermal electric power generation system or the integrated solar combined cycle electric power generation system is used, there exists an unavoidable problem. This problem is associated with the condition of solar radiation onto the surface of the Earth incidentally changing (with time) during daytime. In the solar heat collecting unit, heat transfer from solar heat to steam or other heating medium is mostly based on solar radiation condition. Accordingly, the temperature of steam or other heating medium absorbing solar heat fluctuates in exact response with changes in the condition of sunshine onto the surface of the Earth. As a result, the condition of generated steam (including temperature, pressure, dryness and the like) to be supplied to the steam turbine always fluctuates, which causes the generated electricity to fluctuate. If vigorous fluctuation occurs in the condition of steam, the waste heat recovery boiler or the steam turbine might be damaged thereby.
With the two systems disclosed in patent documents 1 and 2 for example, the condition of steam (including temperature, pressure, dryness and the like) generated in a heat absorbing tube associated with the heat collecting unit fluctuates, thus causing steam to lose heat while being fed from the heat collecting unit to the steam turbine. As a result, the system according to patent document 1 allows the condition of steam to fluctuate after mixing with the steam generated from the high-pressure turbine. The system according to patent document 2 allows the condition of steam to fluctuate at the inlet side of the reheater thereby influencing the waste heat recovery boiler. That is, when the sunshine condition suddenly fluctuates largely or frequently, the condition of steam generated in the heat collecting unit fluctuates likewise, which makes it difficult for the whole of the integrated solar combined cycle electric power generation system to serve continuously for stable and safe operation.
Such fluctuations in sunshine conditions are caused by clouds, sandstorms or the like. When the aforementioned reflector is bent by wind incidentally, sunlight cannot be sufficiently concentrated on the aforementioned heat absorbing tube. This also causes temperature fluctuations of the heating medium or the like. Since such fluctuations possibly occur at short intervals, the aforementioned heat storage system cannot be utilized to suppress effectively the temperature fluctuations of the heating medium or the like.    Patent document 1: European Patent Laid-Open Publication No. 0750730    Patent document 2: European Patent Laid-Open Publication No. 0526816